FEAT Leetspeak converter should have a deterministic option

[romanlutz]

Is your feature request related to a problem? Please describe.

The leetspeak converter is not deterministic and often uses fairly obscure substitutions.

Describe the solution you'd like

The converter should allow me to pass my own substitutions or at least have a "deterministic" flag which selects the most obvious substitutions rather than random ones.

Describe alternatives you've considered, if relevant

Additional context